modf() is a Mathematical Function that converts the "num" into integer, fractional parts. The function returns the fractional part, and places the integral part into the variable pointed by "i".

    float modf( float num, float *i );
    double modf( double num, double *i );
    long double modf( long double num, long double *i );

   #include <iostream.h>
   #include <cmath.h>
   int main()
    {
       double i, r;
       r=modf(9.23, &i);  
       cout << "Value returned by modf is::" << r << endl;
       cout << "Integer part of the number is::" << i << endl;
       return 0;
    }

    Value returned by modf is:: 0.23
    Log10 of 1000 is:: 9

In the above example modf() is used to extract the fractional value alone from "9.23", the integral parts is stored in "i".